This is a protocol to evaluate the effects of tocolytic agents on preterm infants. It has been noted that the incidence of periventricular leukomalacia (PVL) is reduced in infants whose mothers were given MgSO4, rather than another tocolytic agent, during attempts to halt preterm labor. Currently, the three commonly used tocolytic agents are MgSO4, ritodrine and terbutaline. In this prospective study these three drugs will be randomized to mothers in preterm labor. Onset of PVL will be monitored by routine cranial ultrasound, and long-term follow-up will determine whether these children develop cerebral palsy. The CDMAS will create a Web-based program to randomize the mothers to the three treatment drugs.